Optical compensators are used to improve the optical properties of liquid crystal displays (LCD), such as the contrast ratio and the grey scale representation at large viewing angles. For example in uncompensated displays of the TN or STN type at large viewing angles often a change of the grey levels and even grey scale inversion, as well as a loss of contrast and undesired changes of the colour gamut are observed.
An overview of the LCD technology and the principles and methods of optical compensation of LCDs is given in U.S. Pat. No. 5,619,352, the entire disclosure of which is incorporated into this application by way of reference.
As described in U.S. Pat. No. 5,619,352, to improve the contrast of a display at wide viewing angles a negatively birefringent C-plate compensator can be used, however, such a compensator does not improve the greyscale representation of the display. On the other hand, to suppress or even eliminate grey scale inversion and improve the grey scale stability U.S. Pat. No. 5,619,352 suggests to use a birefringent O-plate compensator. An O-plate compensator as described in U.S. Pat. No. 5,619,352 includes an O-plate, and may additionally include one or more A-plates and/or negative C-plates.
The terms ‘O-plate’, ‘A-plate’ and ‘C-plate’ as used in U.S. Pat. No. 5,619,352 and throughout this invention have the following meanings. An ‘O-plate’ is an optical retarder utilizing a layer of a positively birefringent (e.g. liquid crystal) material with its principal optical axis oriented at an oblique angle with respect to the plane of the layer. An ‘A-plate’ is an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis oriented parallel to the plane of the layer, and its ordinary axis (also called ‘a-axis’) oriented perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light. A ‘C-plate’ is an optical retarder utilizing a layer of a uniaxially birefringent material with its extraordinary axis (also called ‘c-axis’) perpendicular to the plane of the layer, i.e. parallel to the direction of normally incident light.
As an O-plate retarder for example an optical retardation film (hereinafter abbreviated as ORF) comprising a layer of a liquid crystal or mesogenic material with tilted or splayed structure can be used.
As an A-plate retarder for example a uniaxially stretched polymer film, like for example a stretched polyvinylalcohol (PVA) or polycarbonate (PC) film, can be used. Alternatively, an A-plate retarder may comprise for example a layer of a positively birefringent liquid crystal or mesogenic material with planar orientation.
As a negatively birefringent C-plate retarder for example a uniaxially compressed polymer film can be used. Alternatively, a negatively birefringent C-plate may comprise for example a layer of a liquid crystal or mesogenic material with a planar orientation and a negative birefringence. Typical examples of negatively birefringent liquid crystal materials are various kinds of discotic liquid crystal compounds.
In addition to U.S. Pat. No. 5,619,352, optical compensators comprising one or more O plates are described in prior art in WO 97/44409, WO 97/44702, WO 97/44703 and WO 98/12584, the entire disclosure of which is incorporated into this application by way of reference. WO 97/44703 and WO 98/12584 further suggest to use O plates in combination with a A plate. WO 97/44703 reports that the use of a compensator comprising a O plate in combination with a A plate, wherein the principal optical axes of both ORFs are oriented at right angles to each other, allows particularly good compensation of a TN-LCD, as it simultaneously reduces the angle dependence of the contrast and the grey scale inversion in the display.
However, when using compensators as described in the above mentioned prior art in combination with liquid crystal displays, especially TN or STN-displays, the improvements of the optical properties of the display, like contrast at wide viewing angles, grey scale level stability, and suppression of grey scale inversion, are still far from sufficient for most applications.
Therefore, it is desirable to have available improved optical compensators to further improve the optical performance of LCDs.